minerals Article
A Mosaic of Colors: Investigating Production Technologies of Roman Glass Tesserae from Northeastern Italy Sarah Maltoni 1 and Alberta Silvestri 2,3, * 1 2 3
*
ID
Department of Cultural Heritage, University of Padova, Piazza Capitaniato 7, 35139 Padova, Italy;
[email protected] Department of Geosciences, University of Padova, via Giovanni Gradenigo 6, 35131 Padova, Italy CNR-ICMATE, Corso Stati Uniti 4, 35126 Padova, Italy Correspondence:
[email protected]; Tel.: +39-(0)49-827-9142
Received: 25 May 2018; Accepted: 12 June 2018; Published: 16 June 2018
Abstract: In the current study, a set of 60 glass tesserae from two disrupted Roman mosaics located in Pordenone and Trento (northeastern Italy) are analyzed, with the aim of investigating the coloring and opacification techniques, with a focus on the causes of specific textural features. All the available colors and textures were selected for archaeometric analyses, in order to guarantee the full characterization of both assemblages and comparisons between the two sites. The applied analytical protocol comprises micro-textural and preliminary chemical characterizations of the tesserae by means of OM and SEM-EDS, mineralogical analysis of the opacifiers by XRD and chemical analysis of the glassy matrices by EPMA; in addition, on specific tesserae, micro-Raman spectroscopy, FORS, and EPR were also performed to clarify the type of opacifer, coloring ion and oxidation state, respectively. Results show that both the base-glass and the coloring/opacification techniques identified are consistent with the presumed Roman dating of the mosaics. All the tesserae are natron-based and chemically comparable with major Roman compositional groups, except for red samples. Antimony-based opacifiers are identified in most of the blue, turquoise, white, yellow and green tesserae, and copper-based opacifiers in the red ones; cobalt and copper are the most frequent ionic colorants used to obtain various shades of blue, turquoise and green colors. Despite the general comparability of both assemblages with the published data on glass tesserae coeval in age, the present study shows differences in the technological solutions used for obtaining the same color, and less common coloring and opacification techniques in three samples from Pordenone. The banded textures of some tesserae were also carefully investigated, and multiple factors influencing the changes in color (different distribution or relative abundance of opacifiers, crystal size, micro-texture, chemical composition of glassy matrix) are identified. Keywords: Roman glass tesserae; micro-texture; coloring; opacification; Italy; SEM; XRD; EPMA; production technology; archaeometry
1. Introduction Glass mosaic tesserae, like the other types of opaque colored glass, are heterogeneous materials, composed of glassy matrix and crystalline phases, and produced from selected raw materials (generally minerals) with various functions (formers, fluxes, stabilizers, colorants, decolorants and opacifiers), by means of pyrotechnological processes. In particular, color is the most important parameter characterizing glass mosaics; the special beauty of a glass mosaic partly depends on how the material is cut and shaped, but more importantly on how the various colors are first obtained and then applied
Minerals 2018, 8, 255; doi:10.3390/min8060255
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near each other in order to create an image. The glass color is determined by a combination of chemical and micro-structural effects. It is mainly due to transition metal ions (e.g., Fe2+ /Fe3+ , Co2+ , Cu2+ , and Mn3+ ), which cause selective absorption of electromagnetic radiation in the visible band and act as coloring agents. Other coloring effects can be produced when a metal is dispersed as minute particles in the glass, the color depending on the size of the colloidal dispersion. Lastly, color may be due to crystalline phases dispersed in the matrix, which also act as glass opacifiers, due to diffusion of incident radiation, generated by refraction between the glass matrix and microcrystalline areas within the glass itself [1]. The scientific analysis so far conducted on Roman and Byzantine glass mosaic tesserae (e.g., [2–13] and references therein) highlights a significant continuity, with some major changes mostly in the opacifiers. The most common opacifiers of the Roman glassmaking are indeed antimony-based (Ca-antimonate and Pb-antimonate) and copper-based crystalline phases (cuprite and metallic copper), although they were used for a very long time (from around 1500 BC until modern times) [14–17]. Two methods of production are hypothesized for Ca-antimonate: the “corpo” method, in which the pigment is synthesized ex situ, and the in situ precipitation that is obtained by adding excess antimony to the molten glass [18]: both methods are attested in Roman and Byzantine production, with a prevalence of the in situ crystallization [19]. In regard to Pb-antimonate, it is commonly accepted that this compound was synthesized ex situ and then added to the molten glass in a very quick process [17,20]. Copper-based phases (cuprite and metallic copper), which are responsible for the orange/red color and opacity of the glass, are always synthesized in situ, if reducing conditions are established in furnaces, and previous studies have been demonstrated that lead and copper play an important role in influencing the type of synthesized phase, and, that texture, size and abundance of crystals may influence hue [15,21,22]. After the 4th century AD the antimony-based opacifiers are progressively substituted by tin compounds (e.g., cassiterite and Pb-stannate), bone ash, and ground quartz, as testified by the Byzantine and medieval mosaics so far analyzed (e.g., [5,11,23,24]). Independently from the age of mosaics, the most common intentional ionic colorants are cobalt and copper for the blue and green [25] and manganese for the purple glasses [26]. Despite the great number of papers dedicated to Roman and Byzantine mosaic tesserae (e.g., [2,3,6,8,10,12,27] and references therein), relatively little attention is still given to the textural features of these artefacts, which can provide useful information on raw materials, temperatures, times of firing, and opacification techniques. In particular, banded tesserae, that are present in Roman and Byzantine mosaics, are underrepresented in the archaeometric literature and, when included, little or no attention is dedicated in investigating the causes of the non-homogenous textures and the generation of the various colors. The present study, analyzing a selection of 60 tesserae from two disrupted Roman mosaics, characterized by a wide range of colors and textures, gives valuable insights into the high technological skills of the Roman glassmakers and in the variety of technological solutions employed in the production of each color. In addition, it aims at expanding the database on Roman glass tesserae from Italy, and the understanding of the textural characteristics of opaque colored glass, with a focus on banded tesserae, in the main perspective of color generation. Complementary analytical techniques (OM; SEM-EDS; XRD; EPMA) are here applied for textural, chemical, and mineralogical study of glassy matrix and crystalline phases in tesserae. These techniques are coupled with other spectroscopic methods (EPR, FORS and micro-Raman spectroscopy), to address specific questions related to the type of coloring ions and opacifiers used in some tesserae. 2. Materials and Methods The glass tesserae investigated here come from two inland cities of northeastern Italy, Pordenone and Trento; both assemblages derive from disrupted mosaic decorations that were excavated in secondary sites.
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The tesserae from Pordenone were excavated in the Roman villa of Torre. The first archaeological investigations were conducted between 1940 and 1952, under the guidance of the owner of the area, the Earl of Ragogna, who was an enthusiastic self-taught scholar [28]. The excavations were conducted in the absence of a stratigraphic approach and, for this reason, tesserae and mosaic fragments cannot be precisely dated. The life span of the villa is wide (1st–5th century AD), but a large quantity of glass tesserae was excavated in the baths, which were in use until 3th century AD, suggesting that an early dating may be hypothesized for the mosaic decoration [28]. The tesserae from Trento were excavated under the main church of Santa Maria Maggiore, and they likely come from the decoration of the pre-existing Roman bath, built in the 2nd century AD and disrupted in the 4th century AD for the construction of the paleo-Christian church. The chronological frame considered for the tesserae is 2nd to 4th century AD, but the mosaic decoration (and consequently the tesserae) are more likely dated to the period of the construction of the baths, i.e., the 2nd century AD [29]. A selection of 60 tesserae (28 from Pordenone and 32 from Trento, labelled with the prefix PN and TN, respectively) of all the available colors, degrees of diaphaneity, and textures (Table 1—Figure 1) was devoted to archaeometric analyses. The selected tesserae were preliminarily grouped by color by means of naked-eye observation, following the colorimetric subdivision, already applied for other assemblages previously studied by authors [7,13,30]. For a more objective color classification, this subdivision was also checked by measuring colorimetric coordinates of all opaque and colored tesserae by FORS. The assemblages of Pordenone and Trento comprise 21 colors, furtherly grouped into 7 color macro-groups (colorless, turquoise, blue, white, yellow, green and red) on the basis of opacifiers and/or ionic colorants/decolorizers identified (Table 1). All the above colors are comparable in both assemblages, except for the light amber tessera identified only in Pordenone and the olive-green and dark amber ones only identified in Trento. The majority of the tesserae are opaque, although some translucent (all the dark blue and olive green, 1 blue, 3 dark green, and 1 yellow-green) and transparent (all the colorless tesserae) ones are also present. In addition, while most of the samples are uniform in color, some banded samples are identified. In detail, some samples, especially those belonging to the macro-group red (but also in the turquoise, blue, yellow, and colorless macro-groups), are banded with areas of darker and brighter color or with areas showing different colors, which can be all opaque (e.g., orange and red), opaque and translucent (e.g., red and green, respectively), or translucent and transparent (e.g., purple and colorless), and, in the case of turquoise, blue and yellow tesserae, by strips of opaque glass in a translucent colored matrix (Figure 1). Where possible, the various glass bands are analyzed separately. This was done in the case of some colorless (samples PN AU1, PN AU2 and TN AU1, composed of supporting tessera, gold foil and cartellina, see Section 3.2 for further details), turquoise (PN TU2), red (PN AV2, TN AV1, and TN M2) and yellow (PN GSO2) tesserae, for a total of 68 analytical samples vs 60 tesserae. A combination of analytical techniques is employed here to guarantee the complete characterization of the samples, composed of glass and crystalline phases, acting as opacifiers. Micro-textural analyses are carried out by means of optical microscopy (OM) and scanning electron microscopy (SEM) coupled with an energy-dispersive spectrometer (EDS), which also allows the qualitative and semi-quantitative chemical analyses of both the glassy matrix and inclusions. Optical microscopy observations were conducted under stereoscopic vision on the whole tesserae with a Zeiss Stemi 2000C microscope (Carl Zeiss, Oberkochen, Germany) and under reflected and transmitted light on the polished sections with a Nikon Eclipse Me600 (Nikon Corporation, Tokyo, Japan). The instruments employed for SEM-EDS analyses are an ESEM FEI Quanta Inspect equipped with an Oxford energy dispersive spectrometer and a FEI Quanta 200 FEG-ESEM instrument equipped with a Genesys energy-dispersive X-ray spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). In both instrumentations, SEM images were taken by collecting the backscattered electron signal (BSE), operating under high-vacuum conditions (5 wt % as PbO) is also identified in some tesserae of the red, yellow and green macro-groups. The present results show how the presence of lead is linked to the production of specific colors, such as red, green and yellow, and that very high lead (>20 wt % as PbO) is specifically used to produce orange tesserae. Further discussion on the role of lead in orange glass is presented in the sub-section on “red tesserae”. In order to determine the type of base glass used to make the tesserae from Pordenone and Trento, a so-called reduced composition of each glassy matrix is calculated, following the method reported in [11], because the intentional addition of lead, colorants and opacifiers (discussed in the next sub-sections) affects the original chemical compositions of tesserae, sometimes considerably. The reduced compositions are then compared against literature compositional groups for the period of interest; in particular, the main compositional groups dominating the Roman period (1st–3rd century AD) are Sb-colorless, Mn-colorless, Sb/Mn colorless, and unintentionally colored glass. They differ in antimony and manganese contents, being Sb-colorless characterized by high antimony content (Sb2 O3 = 0.81 ± 0.16 wt %) and no manganese, Mn-colorless by very high manganese (MnO = 1.41 ± 0.27 wt %) and no antimony, Sb/Mn colorless glass by the presence of both manganese (MnO = 0.41 ± 0.16 wt %) and antimony (Sb2 O3 = 0.43 ± 0.15 wt %), and unintentionally colored glass by generally low manganese (MnO < 1 wt %) and negligible antimony [37–39]. In addition to different ratios of manganese and antimony, which are not considered in the reduced compositions, the other key characteristics of these groups are the different contents of SiO2 , Na2 O, CaO and Al2 O3 [37,39]: Sb-colorless glass is characterized by high silica and soda (SiO2 : 69 ÷ 73 wt %; Na2 O: 18.5 ÷ 20.5 wt %) and low lime and alumina (CaO: 4 ÷ 5.5 wt % and Al2 O3 : 1.6 ÷ 2.2 wt %), Mn-colorless and unintentionally colored glass by low silica and soda (SiO2 : 67 ÷ 71 wt %; Na2 O: 14.5 ÷ 17 wt %) and high lime and alumina (CaO: 7 ÷ 9. wt % and Al2 O3 : 2.5 ÷ 3 wt %), and Sb/Mn colorless glass by intermediate contents of the above elements (SiO2 : 68.5 ÷ 71 wt %; Na2 O: 16.5 ÷ 18.5 wt %; CaO: 5.5 ÷ 7 wt %; Al2 O3 : 2 ÷ 2.5 wt %). Therefore, the Roman compositional groups are here referred as Sb-glass, Sb/Mn glass and Mn-glass (the last including both Mn-colorless glass and unintentionally colored glass).
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Table 2. The mean chemical compositions of the glass tesserae (EPMA data, expressed as weight percentage of the corresponding oxide, except for chlorine; standard deviation in italics). Color macro-group, color, diaphaneity reported for each sample. Sample
Color Macro-Group
Color
Diaphaneity
SiO2
Na2 O CaO
Al2 O3 MnO Fe2 O3 MgO K2 O
TiO2
P2 O5
SO3
Cl
CoO
CuO
ZnO
SnO2 Sb2 O3 PbO
TOT
PN AU1
colorless
gold
transparent
100.76
